Rubber modified styrene resins such as HIPS (high impact polystyrene) are now widely used as thermoplastic resins having excellent impact resistance. Such rubber modified styrene resins represented by HIPS generally have a dispersed rubber particle diameter as large as 1 .mu.m or more and as a result they have a defect that their molded products have a poor luster.
Accordingly, development of resins that have both acceptable luster and the impacts resistance of HIPS has been promoted. For example, Japanese Unexamined Patent Application, First Publication No. Hei 5-1122 discloses a technology of reducing the particle diameter of grafted rubber particles in HIPS to 0.5 to 1.0 micron to impart both luster and impact resistance thereto.
Also, Japanese Unexamined Patent Application, First Publication No. Hei 4-180907 discloses a technology of providing a resin composition that has excellent transparency, rigidity and impact resistance by graft polymerizing styrene and methyl methacrylate with a rubber component.
However, in the technology described in Japanese Unexamined Patent Application, First Publication No. Hei 5-1122, although a reduction in rubber particle diameter increases the luster to some extent, the reduction in particle diameter is accompanied by a serious decrease in impact strength, so that in order to obtain products having a decreased rubber particle diameter and high impact strength, the rubber content in the product must be higher than in common HIPS, and as a result rigidity, which is one of the characteristic features of styrene resins, is decreased. Further, use of more rubber component with smaller particle diameters has a problem regarding impact strength that the resultant composition is low in practically important surface impact strength but is high in notched Izod impact strength.
Also, the resin composition described in Japanese Unexamined Patent Application, First Publication No. Hei 4-180907 has transparency so that it has a good luster but in order to impart transparency, the particle diameter must be decreased further and as a result the resultant composition has an insufficient impact strength, thereby finding only limited applications.